It is known to use time proportional control for on/off triac circuit arrangements for controlling the temperature of heating elements in hotplates, frypans, irons and the like. In these circuit arrangements the temperature of the element is sensed by a separate negative temperature coefficient (NTC) or separate positive temperature coefficient (PTC) thermistor thermally coupled to the object to be controlled. Effective thermal coupling between the sensor and the heating element is difficult to achieve in high temperature applications and improper thermal coupling results in large temperature errors. Alternative methods of sensing the temperature of the element rely on using the temperature coefficient of resistance (TCR) of the element itself.
U.S. Pat. No. 5,847,367 discloses as a mere paper publication a circuit for controlling heating elements that exhibit a high “positive temperature coefficient” of resistance (PTC) i.e. its ohmic resistance value increases with increasing temperature.
For example it is known that pure metals exhibit a positive temperature coefficient, whereas alloy resistance elements generally seek to have no temperature coefficient. In the circuit arrangement discussed in the above patent, a separate current sensing means connected in series with the element is used to measure the current through the element. The current flowing through the element is determined by its resistance, which in turn is a function of its temperature.
Thus there is no need for a separate thermocouple sensor. The circuit is supplied by an alternating current mains supply coupled to the element via a switching means. In this circuit when the temperature of the heating element becomes too hot, the alternating current flowing through the element is switched off. When this occurs it is not possible to subsequently measure the temperature of the heating element.
In order to subsequently measure the temperature of the heating element the circuit has a timer which periodically switches on the element for a period determined by the controller. The circuit uses a comparator to compare the voltage provided by the sensing means with the voltage on a variable resistor to determine whether the temperature of the element is above or below the temperature set by the user. As the voltage supplied by the current sensing means is applied directly to the comparator the circuit requires large changes in the resistance value of the element with temperature in order to closely determine the variation of the temperature of the element from the desired temperature.
Investigations have shown that this circuit has a number of limitations. In certain applications where the PTC characteristic of the basic element material is very low, the circuit does not have sufficient discrimination to accurately control the temperature of the element. Also in such applications, because of the low PTC characteristic a small error in absolute resistance is equivalent to a large temperature error. This circuit has no means to compensate for tolerances in absolute resistance value of the element.
In all cases fluctuations in mains supply voltage will vary the current through the element, and also through the sensing means. The circuit reacts to changes in the current detected flowing through the sensing means by increasing or decreasing the temperature of the heating element to compensate. Therefore the accuracy of the regulation of the temperature of the heating element is dependent on mains supply voltage.